Does anybody knows any economical calculations or reasons how large should be floor area for highrise buildings to become profitable?

I've red somewhere that the first law of high-rise buildings is that they become profitable, with a floor space of over 1500 square metres. I do not kow is it said about offices only or not.

So such a building with a 1500 square metres per floor should be pretty tall to look slender.

It strongly depends on the geographic location where you are building a highrise.
You have got factors like regulations and restrictions applied by the local law, the local real estate prices and rents, the design, the local demand to space or quality or quantity. These are all factors, and many more, which play a role in the calculations wheter a building is profitable or not. As far as I know, there is no such formula which can be applied to all buidlings. There are model calculations. But those are specified to certain types of building.
But the most profitable floor space is a dynamic output that has to be calculated trough all factors which play a role. Then it becomes though mathematics ;)

There is absolutely nothing decent to say about any gfa / profitablity relationship because there are too may variables involved.

Let me try to anwer it in this manner.

In general The higher the efficiency (ratio of end usable space to non usable or common areas)of your floorplate the better the chances of profitability in a project.

As hi-rises go, the higher it gets the more space needed to take up the elevators , fire escapes and the like. This is one of the biggest challenges of architects/engineers in very tall buildings reconciling safety transport issues .

There is a point wherein the efficiency of hi-rises start going down as architects start making space for more and more elevators and the like. This is the point where the maxims of profitablity, and number of floors must be optimised.

Of course new technology may keep pushing and pushing the limits higher. Definitely even just 15years ago you would not have seen as many profitable 30 -40 -50 story residential building in Asia as you see now. This is the result of better technology making the efficiency needed for a profitable project possible.

would it still be the same with commercial and residential tenants? or is that why we are seeing more mixed use buildings.

I think mixed-use has pros and cons. Having a variety of uses means that you are addressing a variety of markets thus spreading the risk. Look at LBT for example - they now have a hotel tennent signed up, reducing the need for them to find a large scale pre-let in the office space. They know they can shift the residential aspects of the scheme so their sums now add up and they can start construction even before a single office occupier has signed up.

However it does create difficulties in internal organisation, complicates various regulations with regards to fire and so forth and I assume must also make it much more untidy for the landlord in terms of the variety of leases they have to deal with.